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Identifying Small, Medium, and Worst-Case Discharge Scenarios

Posted on Thu, Aug 02, 2012

Responding to an oil spill can be complex with many factors involved. As a result of the Oil Pollution Act of 1990 (OPA 90), marine vessels and certain land-based facilities that handle, store, or transport oil are required to have response plans detailing response actions. This regulation mandates that owners or operators of applicable facilities prepare and submit response plans for responding to a worst-case discharge.

The Environment Protection Agency (EPA) states that site-specific scenarios and response resources must be addressed for small, medium, and worst-case spills. A smaller facility may only need to plan for two scenarios or a single scenario if its worst-case discharge falls within one of the specified ranges for small or medium discharges.

Note- “Owners or operators of marine-transportation-related facilities with animal fats and vegetable oils must plan for a worst case discharge and an Average Most Probable Discharge (the equivalent of a small discharge in the EPA rule). The response plan requirements differ primarily because EPA -regulated non-transportation-related facilities generally are much larger than the USCG-regulated marine transportation-related (MTR) facilities and often have a potential for worst case discharges that are greater by an order of magnitude or more.” -Environmental Protection Agency

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Discharges are categorized by the following volumes:

  • Small discharge: up to 2,100 gallons spilled
  • Medium: 2,100 to 36,000 gallons spilled, or 10% of the largest tank (whichever is less)
  • Worst Case Discharge: Volume of the largest tank over 36,000 gallons

The source of a small, medium, and worst-case discharge may stem from various facility operations and corresponding equipment components.  Potential discharge scenarios can be derived from human error, equipment malfunction, third party intervention, or severe weather. Typical site components relating to discharge scenarios include, but are not limited:

  • transfer hose failure
  • Improper or faulty hose seals
  • valve failure
  • misaligned piping connection or seal failure
  • pump seal failure or overfill
  • tank overfill or leak
  • catastrophic failure of largest tank

Most spill scenarios would likely be contained in specified areas or by specialized equipment, and are unlikely to travel off site. However, if the scenario created could potentially result in oil traveling off site, its migration pattern, potential traveling distance, and specifically identified locations should be detailed.

Any spill response, despite the size of the spill, should respond to these types of incidents in the same manner as a worst-case discharge, incorporating the company defined preparedness structure and procedures. Despite the voluminous details and the nature of a spill, all employees and responders should demonstrate an understanding and application of company policies and agency requirements through an established training and exercise cycle.

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Tags: EPA, OPA 90, Oil Spill, Disaster Response